Establish a variable inside a driver expression
The driver namespace

Can add functions and constants to the driver namespace

import bpy

def func(a, b, c):
    return a * b * c

m = 9.4443299912

bpy.app.driver_namespace["f"] = func
bpy.app.driver_namespace["m"] = m


Run the script above, (copy &amp; paste into text editor hit Run Script button,)  to make the new members available to the driver namespace. To make it autorun when file is loaded, give the text block a py extension and check the Register checkbox on RHS of text editor header



Can now consider m in a driver expression to be a named constant like pi and e.

Use in a driver expression like f(m, m, m) Can be typed directly into a field by preceding with a hash # character.  Once the expression is a driver (purple) on editing expression there is no longer a need for #.



There are a number of functions and properties etc already defined to the namespace, a sample of python console output.

&gt;&gt;&gt; for k, v in bpy.app.driver_namespace.items():
...     k, v
...     
('asin', &lt;built-in function asin&gt;)
('asinh', &lt;built-in function asinh&gt;)
('atan', &lt;built-in function atan&gt;)
...
('pi', 3.141592653589793)
('e', 2.718281828459045)
...
('f', &lt;function func at 0x7f8f18d34c80&gt;)
('m', 9.4443299912)
('frame', 1.0)


Where frame is the context.scene.frame_current.

EDIT: would like to see a "sub expression" driver variable type where you could do this for scalars, or using other defined variables.  Fitting everything into the scripted expression can be a PITA.